The invention relates to DC motor controls, and more particularly to those controls which include a motor field regulator circuit for regulating the current supplied to a field winding of a DC motor.
The outstanding characteristic of a DC motor is its adaptability to control of its torque and speed. The typical DC motor comprises two primary parts, a field which is stationary, and an armature which rotates. The field may be either a permanent magnet or an electromagnet, and its purpose is to establish magnetic poles on the frame of the motor, which act in conjunction with the magnetic poles of the armature to provide the forces that cause rotation of the armature. Magnetic flux is induced in the poles of a field electromagnet pole by passing a field current through a field winding which is wrapped in numerous turns around the poles.
The armature consists of a plurality of windings, which are connected to terminals on the motor through a commutator and brushes. The commutator serves to switch power to successive coils as the armature rotates, so that the magnetic armature poles remain in the same location relative to the field poles.
The horsepower output of a DC motor in electrical terms is proportional to (1) the current through its field winding, (2) the current through its armature and (3) the rotational speed of the armature. This can be expressed in an equation as follows: EQU H.P.=KI.sub.F I.sub.A S (1)
where:
H.P.=horsepower PA1 I.sub.F =field current PA1 I.sub.A =armature current PA1 S=rotational speed of the armature PA1 K=a constant PA1 H.P.=horsepower PA1 T=output torque PA1 S=rotational speed of the armature
The horsepower output of the motor--in mechanical terms--is provided by the product of (1) output torque and (2) the rotational speed of the armature. This can be expressed in an equation as follows: EQU H.P.=TS (2)
where:
The right sides of equations (1) and (2) can be equated and the rotational speed of the armature cancelled from each side to produce: EQU T=KI.sub.F I.sub.A ( 3)
As a result of these relationships, the operation of a DC motor can be summarized by referring to two basic ranges of speed control. The first range is known as the "constant torque" range and the second range is referred to as the "constant horsepower" range. In the "constant torque" range the ability of the motor to deliver torque to a load is not affected by an increase in speed, because it has not yet reached its maximum horsepower or armature current. Thus, for example, if the field winding carries a full rated field current, the speed of the motor can be increased by varying the applied armature voltage. Speed will increase with armature voltage until the motor reaches a speed known as the "base speed" which corresponds to the speed at which rated horsepower is obtained at rated armature current and rated armature voltage of the motor.
If it is desired to operate the motor above the base speed, it becomes necessary to weaken the field, since further increases in armature voltage are not permitted by the maximum ratings of the motor. From equation (3) it will be seen that a reduction in field strength and field current will be accompanied by a proportionate reduction in available maximum torque. As seen from equation (2) above, the decrease in torque will allow an increase in speed if horsepower remains constant. There are a number of loads that can be controlled with reduced torque, but which require speeds higher than the base speed. It is therefore advantageous to provide a motor control that will operate at speeds higher than the base speed. This speed range is known as the "constant horsepower" range of operation.
One prior approach to weakening field strength for operation in the "constant horsepower" range utilized circuitry that responded to changes in measured armature voltage. This response was considered to be a slow and somewhat coarse adjustment. The reason that such an adjustment is not as fine as desired, is that the voltage measured across the armature (V.sub.T) is not always a true measure of the voltage generated by a DC motor. The voltage generated by the motor is referred to as the counter-EMF, since its polarity is opposite to the applied voltage.
When the DC motor is operated in a motoring mode, the armature voltage (V.sub.T) is equal to the sum of the motor counter-EMF and the voltage drop due to current flow in the armature windings. The DC voltage drop can be expressed as the product of the DC armature current (I.sub.A) and the resistance of the armature (R.sub.A). The result is the following two expressions: EQU V.sub.T =CEMF+I.sub.A R.sub.A ( 4) EQU CEMF=V.sub.T -I.sub.A R.sub.A ( 5)
When the DC motor is operated in the regenerating mode, the polarity of the armature voltage (V.sub.T) and the polarity of the counter-EMF remain the same, but the polarity of armature current (I.sub.A) is reversed to provide the following two expressions: EQU V.sub.T =CEMF-I.sub.A R.sub.A ( 6) EQU CEMF=V.sub.T +I.sub.A R.sub.A ( 7)
The counter-EMF is a function of field strength. In some instances, a reduction in field current will not cause a proportionate reduction in field strength, due to operation of the field electromagnet in its nonlinear range. The counter-EMF is also a function of speed, so an increase in speed above the base speed and a decrease in field current will satisfy equation (1) above, but it will result in an excessive counter-EMF. At "no load", the I.sub.A term in equations (4)-(7) is zero, and the voltage measured across the armature (V.sub.T) is equal to counter-EMF. Under load conditions, however, this is no longer true. Because the prior approach to weakening field strength did not compensate for the IR voltage drop under load conditions, it provided only a coarse adjustment to field current in response to armature voltage.